1. Field of the Invention
This invention relates to methods of and circuits for video signal processing. More particularly, the invention is concerned with distinguishing between the front and the back of an image corresponding to a video signal which has been subjected to special effects processing. Still more particularly, but not exclusively, the invention relates to video signal processing circuits for so distinguishing and which are suitable for use in special effects equipment in high definition video systems.
2. Description of the Prior Art
The standard television signal transmitted in the United Kingdom is a PAL signal of a 625-lines per frame, 50-fields per second system, and the PAL, NTSC and SECAM signals transmitted in other countries use similar or slightly lower line frequencies (for example 525 lines per frame), and similar or slightly higher field frequencies (for example 60 fields per second). While there is no immediate prospect of significant changes in these transmitted signals, there is an increasing requirement for higher definition video systems. Such systems can be used, for example, in film-making, in closed circuit television systems, in satellite communication systems and in studio use generally. One such proposed high definition video system uses 1125 lines per frame and 60 fields per second. This proposed system also uses a 5:3 aspect ratio instead of the 4:3 aspect ratio now usual for television receivers.
The special effects which can be applied to a video signal are well known. Thus, for example, images on a cathode ray tube can be off-set (moved in any direction), scaled (expanded or compressed in size), rolled (rotated) in two or three dimensions and so on.
One way of achieving such special effects, which will be referred to in more detail below, involves converting an input analog video signal into digitized sample values each having a pixel address, modifying the resulting individual pixel addresses to achieve the required special effect, storing the sample values at the modified pixel addresses in a field memory, and reading from the field memory to derive the sample values for reconversion into the required output analog signal.
The effects to be achieved by special effects equipment can in general be divided into two types: those which do not bend or twist the image plane, that is to say linear effects, which may nonetheless be three-dimensional, and those which do distort the image plane by projecting the image onto a three-dimensional shape, that is to say non-linear effects. An example of a three-dimensional linear effect is tilting the input image plane with perspective, as in a tumble or flip. An example of a three-dimensional non-linear effect is the projection of the input image plane onto the surface of a cone.
Two of the processes involved in producing three-dimensional effects whether linear or non-linear are transformation of the initial two-dimensional pixel addresses to pixel addresses in three-dimensional space, and then perspective transformation back onto the two-dimensional viewing plane.
For linear effects, the required two or three-dimensional pixel addresses can be derived by matrix calculation as used, for example, in computer graphics. However, substantial modification of the techniques is necessary to achieve operation in real time as required in a television system. This forms the subject of our copending UK application No. 8511649 (European application No. 86303298.3). For non-linear effects, there is a requirement for methods and circuits which can not only achieve the required effect, but can also do so at the required speeds and without requiring hardware which is too extensive or too complex. This forms the subject of our copending UK application No. 8511648 (European application No. 86303298.3).
In both linear and non-linear three-dimensional effects the possibility exists of the back of the image becoming visible to a viewer. Thus if the input image plane is progressively flipped over, the back of the image will eventually become visible. Likewise if the input image plane is progressively curled into the shape of a hollow cylinder, the back of the image will become visible on the outside of the cylinder, or on the inside of the cylinder if the axis of cylinder is tipped, depending on whether the direction of the curling. What is actually to be seen will depend on whether the image is taken to be transparent, in which case the back of the image will be the same as the front but will appear reversed, or whether the image is taken to be opaque with another image arranged back-to-back with it, in which case the other image will become visible. In either case it is necessary to determine whether the front or the back of the input image will be seen by a viewer.
It might be thought that before transforming the image to achieve the required three-dimensional effect, a set of vectors could be defined normal to the image plane and pointing towards the viewer. After the transformation these vectors could be used to determine whether the front or the back of the image would be visible from the perspective point, by determining the angles of the vectors to the viewing plane. However, such a method would mean that in addition to transforming the pixel addresses of the input image to achieve the required effect, the vectors would also have to be transformed, and this would necessitate substantial additional storage and computation.